Mirrors for changing the geometrical form of a beam are widely known. It is difficult to generate a beam cross section which has a configuration which is essentially linear to rectangular and within which the intensity of the beam has a predetermined course, especially is constant in the longitudinal direction. Such beam cross sections are necessary especially for conducting surface treatment with a laser to name only one essential area of application.
My copending application Ser No. 07/788,997, filed Nov. 7, 1991, which is a continuation-in-part application of my patent application Ser. No. 07/505,177, filed on Apr. 5, 1990 and now abandoned, discloses a mirror of the kind referred to above and is incorporated herein by reference.
The transforming optics assure that the laser beam has a cross section which is rectangular to linear when impinging on the work surface and that the intensity within this cross section has almost a constant value. If a beam with this kind of cross section is guided over the workpiece at constant speed and parallel to a pair of edges of the rectangular cross section, then the energy profile within the irradiated strip is uniform.
The lasers used most in practice supply a beam having a cross section which is not rectangular and whose intensity distribution is not uniform. For this reason, optical arrangements are needed which can suitably transform any beam cross section.
Facet mirrors and integrators are two types of transforming optics which are preferably utilized for generating linear geometries with respect to the intensity distribution of laser rays.
Japanese patent publication 63-77178 A discloses a facet mirror having a plurality of planar mirrors which are arranged so as to lie tangentially to a paraboloidal surface in a mosaic-like manner and which concentrate a laser beam on a rectangle having essentially the extent of an individual planar mirror. However, a line profile is not obtainable in this way since the individual mirrors have no focussing effect. The manufacture of such an arrangement is complex.
Facet mirrors are also disclosed in an article entitled "A Convex Beam Integrator" by Stanley L. Ream, published in "Laser Focus", November 1979, pages 68 to 71. Multi-facetted mirrors are also disclosed in U.S. Pat. No. 4,518,232 and in Japanese patent publication 59-151 101 A with the latter having spherical facets.